example.ts

type DeliminatedDocument = {
  text: string;
  separator: "comma" | "tab";
}

type PlaintextDocument = {
  text: string;
}

type WeirdPlaintextDocument = PlaintextDocument & {
  separator: 'lol';
}

const printDeliminated = (doc: DeliminatedDocument) => {
  console.log('Printing deliminitated document with separator', doc.separator);
}

const printPlaintext = (doc: PlaintextDocument) => {
  console.log('Printing plaintext document');
}

const printDocument = (doc: DeliminatedDocument | PlaintextDocument) => {
  if ("separator" in doc) {
    printDeliminated(doc);
  } else {
    printPlaintext(doc);
  }
}

const handleWeirdPlaintextDocument = (doc: WeirdPlaintextDocument) => {
    printDocument(doc);
}

handleWeirdPlaintextDocument({
  text: 'I am plaintext',
  separator: 'lol',
});